The invention relates more particularly to a method for controlling a molding unit for the forming of a container made of thermoplastic material by blow molding, in particular by stretch blow molding, of a preform, with the molding unit comprising:                A carriage that carries, in a stationary manner, a mold bottom and that is mounted to move in a stationary support between a lower position for extraction of the finished container and an upper position for molding a base of the container;        First actuating means for moving the carriage in relation to its stationary support between the lower extraction position and an initial intermediate boxing position in which the bottom can be enclosed between two half-molds in the attached position;        Second actuating means for moving the carriage in relation to its stationary support from the initial intermediate boxing position to the upper molding position, with the second actuating means comprising at least one stationary cylinder in relation to the support housing a piston that divides the cylinder into a lower working chamber and an upper chamber, with the piston being able to slide between a lower rest position and an upper boxing position in which it pushes the carriage to its upper molding position by pressurizing a working fluid in the lower working chamber relative to the pressure prevailing in the upper chamber;        Third actuating means for moving the carriage from its upper molding position to its lower extraction position;        
With the method comprising a step for extraction of a finished container taking place after the carriage has been pushed to its upper molding position by the piston, during which the carriage is moved from its upper molding position to its lower extraction position by the third actuating means.
Such molding units comprising a device for carrying out a so-called “boxing” operation of the base of a container, such as, in particular, a bottle made of thermoplastic material, are known in the state of the art.
Such a boxing operation is described in, for example, the document FR-2,508,004, to which reference will be made, in a non-limiting manner, for more ample details on this forming technique.
The boxing operation corresponds to a forming technique used during the manufacturing of containers for which particular characteristics are desired, in particular made of material with structural rigidity of their bases, for example for certain plastic bottles manufactured from PET (polyethylene terephthalate).
This forming technique is in particular, but not exclusively, used for the purposes of manufacturing containers such as those bottles that are designed to be filled “hot,” i.e., filled by a liquid to be packaged whose temperature is, for example, between 60° C. and 95° C. It makes it possible to obtain a flexible base forming a membrane that becomes deformed under the action of negative pressure created by the cooling of the hot liquid contained in the closed container.
This technique is also able to be used for reducing the quantity of material present in the base of the container whose weight is desired to be lightened so as to reduce its cost and/or also for aesthetic reasons.
The boxing technique consists in moving, during the forming, the part of the mold that is intended for the molding of the base of the container, called “mold bottom” in the description below, or else “mold of the bottom” in the document referenced above. This movement is carried out while the constituent material of the preform or the container (formed or in formation) also has a suitable deformation temperature. This movement makes it possible for the mold bottom to stretch the material constituting the base of the container. The mold bottom thus emerges like a first for “boxing” the base of the container during forming, hence the name of the boxing operation.
In the known molding installations that do not implement a boxing operation, it is already known to move the mold bottom between its lower position for extraction of the container and its molding position by means of a cam actuating device. Such a device makes it possible in particular to obtain fast and precise control of the movements of the mold bottom. Such a rapidity of actuation is not possible with a pneumatic jack. Nevertheless, the cam actuating devices do not make it possible to obtain a sufficient force to carry out a boxing operation.
To solve this problem, it has been proposed to control the movements of the mold bottom by means of a first cam actuating device making possible fast and precise positioning of the mold bottom, and by means of a second pneumatic actuating device that makes it possible to move the mold bottom on its boxing path with a sufficient force to carry out the boxing operation.
FIGS. 5 to 8 of the document FR-2,508,004 show the primary steps of this technique and illustrate an example of a boxing operation implemented during the forming of a bottle made of thermoplastic material.
By comparison with a conventional mold comprising two half-molds and a separate mold bottom, the mold bottom is not axially immobilized by the attached half-molds but it can be moved axially by actuating means between an initial intermediate boxing position (bottom position) and a final upper molding position (top position).
In FIG. 5 illustrating a first step, the mold bottom is moved toward the half-molds for occupying the initial position in which it is positioned axially set back in such a way that the axial dimension (height) of the molding cavity is larger than that of the body of the final container.
A preform, introduced into the mold, extends axially through an opening emptying into a molding cavity that delimits the two half-molds associated with the mold bottom, said half-molds mounted to move between a separated position and an attached position (shown in FIG. 5).
In FIG. 6 illustrating a second step, the preform is stretched axially by a stretching rod until reaching approximately the top part of the mold bottom.
The mold bottom then again occupies its initial intermediate boxing position; the axial stretching of the preform is done according to a maximum stretching path that is higher than the height of the container (bottle) or with a path that corresponds to the difference between the initial intermediate boxing position and the final molding position of the mold bottom. In this final molding position, the axial dimension of the molding cavity corresponds to that of the final container.
Such a stretching operation is carried out in combination with a blow-molding operation, preferably respectively preceded by a pre-blow molding, in such a way as to obtain a bi-axial stretching, i.e., a biaxial molecular orientation of the material thanks to which the structural rigidity of the base of the container in particular is improved.
At the end of this second step, the base of the container is not yet formed here, and the material still has a deformation temperature. The mold bottom is then moved axially upward by associated actuating means, from the initial position to the final molding position illustrated in FIG. 8.
The mold bottom passes through an axial boxing path during which said mold bottom will stretch the material by axially pushing back the base in the direction of the inside of the container.
However, according to the application, the techniques vary and the boxing operation can be performed during or after the formation of the container that is ultimately shaped by a blow-molding operation at a blow-molding pressure.
The final blow-molding pressure can reach values on the order of 20 to 40 bars according to the applications, or consequently values that are commonly considered to be high pressures.
With the exception of the boxing operation, the forming of the container is consequently carried out in a conventional manner by the stretch blow molding of a preform made of thermoplastic material and thermally packaged in advance for this purpose, with one or more blow-molding steps consisting in injecting a pressurized fluid, in general air, inside the preform.
Of course, the description that was just given is in no way limiting, and various variants or enhancements can be provided thereto to carry out the boxing operation.
To carry out such a boxing operation of the base of the container during its forming, a device of the type described above equips each container molding unit of the machine.
Such a molding unit comprises actuating means that are designed to move the mold bottom selectively between said initial intermediate boxing position and final molding position.
The document FR-2,945,469 describes an example of actuating means of such a molding unit that can carry out a boxing operation.
According to this document, the actuating means of the mold bottom consist of a simple jack operating by means of a pressurized fluid formed by compressed air. The pneumatic jack is carried by a carriage that makes it possible to move the mold bottom from a lower extraction position to its initial intermediate boxing position. The jack is then actuated to move the mold bottom on its boxing path.
The device for carrying out the boxing according to this document and in particular the actuating means formed by such a jack are not, however, fully satisfactory.
Actually, such a device is particularly heavy, and it does not make it possible to manufacture the containers in series at a pace that is as fast as for containers manufactured without a boxing operation. This reduction in pace is due in particular to the heavy weight of the carriage thus produced.
To solve the above-cited drawbacks, a molding unit equipped with a carriage that carries the bottom in a stationary manner has been proposed. The carriage is mounted to move in a stationary support between a lower position for extraction of a finished container and an upper molding position. The carriage can be moved by three actuating means that are arranged in parallel between the stationary support and the carriage.
The second actuating means are formed by a piston that makes it possible to control the sliding of the carriage from an initial intermediate boxing position to the upper molding position. The piston is accommodated in a stationary cylinder in relation to the support that divides the cylinder into an upper chamber and into a lower working chamber. The piston is thus mounted to slide in a stationary cylinder in relation to the support between a lower rest position and an upper boxing position in which it pushes the carriage into its upper molding position by pressurizing the lower working chamber.
The third actuating means are formed by a cam control device, comprising in particular a cam follower roller that is mounted to rotate on the carriage. These third actuating means make it possible to make the carriage slide from its upper molding position to its lower extraction position.
During a step for extraction of a container that takes place after the piston has pushed the carriage to its upper molding position, the carriage is called toward its lower extraction position to allow the extraction of the finished container. The actuating force required on the part of the third actuating means for bringing the carriage back to its lower extraction position should be the lowest possible.
Actually, in a known manner, the third actuating means comprises a safety device that is designed to avoid damage to the molding unit when the sliding of the carriage is blocked. For example, the rod being used as an axis of rotation of the roller is produced in such a way as to break when too large a force is imposed on the roller. Because of this safety device, the service life of the roller and its shaft are relatively short. To extend the life expectancy of the roller and thus to save on replacement material and maintenance time, an effort is made to reduce the actuating force applied by the roller on the carriage during the extraction step.
In addition, the reduction of the actuating force also makes it possible to save energy.
To minimize the actuating force required, one skilled in the art believed it obvious to bring the piston back into its lower rest position before the third actuating means calls for the sliding of the carriage toward its lower extraction position.
To do this, the pressure in the lower working chamber is lowered and/or the pressure in the upper chamber is increased. When the pressure in the lower working chamber becomes less than the pressure in the upper chamber, the piston slides to its lower rest position. During this time, the carriage continues to be held in its upper molding position by an elastic return force. Then, the roller, coming into contact with the cam, causes the carriage to slide to its lower extraction position against the elastic return force. During the sliding, the roller exerts on the carriage a determined maximum actuating force that will be termed “reference actuating force” in the description below.
This way of proceeding in carrying out the extraction step is, at first glance, considered by one skilled in the art as making it possible to achieve the lowest actuating force. Actually, the roller acts only on the carriage.
The invention proposes a method for controlling the molding unit that makes it possible to carry out the extraction step with an actuating force that is also less than the reference actuating force.